The methods of nitriding or carbonitriding steel articles or works for the formation of a nitrided layer on their surface which have been so far employed for the purpose of improving their mechanical properties, such as wear resistance, corrosion resistance and fatigue strength, include the following, among others:
(a) The method of using a molten cyanate or cyanide salt, such as NaCN or KCN (tufftriding method);
(b) The glow discharge nitriding method (ionitriding method); and
(c) The method using ammonia or a mixed gas containing ammonia and a carbon source, for example RX gas (gas nitriding or gas soft nitriding method).
Among these, method (a), which uses hazardous molten salts, has a dark future when evaluated from the labor environment, waste treatment and other viewpoints. Method (b), which achieves nitriding by means of glow discharge in an N.sub.2 +H.sub.2 atmosphere under a low degree of vacuum, can indeed avoid, to a considerable extent, the staining of the steel surface or the influences of oxidized layer formation owing to some cleaning effect of sputtering but tends to allow occurrence of uneven nitriding due to local temperature differences. In addition, this method is disadvantageous in that articles or works which can be nitrided are much limited in shape and size and that increases in cost result Method (c) also has problems, for instance, the treatment process is not very stable but tends to lead to uneven nitriding Another problem lies in that deep nitriding requires a fairly long time.
Generally, steel is nitrided at temperatures not lower than 500.degree. C. For the adsorption and diffusion of nitrogen on the steel surface layer, it is desired that the surface should be free not only of organic and inorganic contaminants but also of any oxidized layer or adsorbed O.sub.2 layer It is also necessary that the steel surface layer itself should be highly active. The above-mentioned oxidized layer, if present, would unfavorably promote dissociation of the nitriding gas ammonia. In practice, however, it is impossible to prevent oxidized layer formation in gas nitriding. For instance, even in the case of case hardened steel or structural steel whose chromium content is not high, thin oxidized layers are formed even in an high concentration hydrogen atmosphere or an NH.sub.3 or NH.sub.3 +RX atmosphere at temperatures not higher than about 500.degree. C. This tendency becomes more pronounced with steel species containing an element or elements which have high affinity for oxygen, for example chromium, in large amounts. Works made of this kind of steel must be deprived of inorganic and organic contaminants prior to nitriding by degreasing with an alkaline cleaning solution or washing with an organic solvent such as trichloroethylene. However, in view of the recent regulations against environmental pollution (regulations against destruction of the ozone layer), the use of organic solvents with highest cleaning effects should be avoided and this is another problem.
The oxide formation on the steel surface, such as mentioned above, varies in extent depending on the surface state, working conditions and other factors even in one and the same work, resulting in unevenly nitrided layer formation. For example, in the typical case of work hardened austenite stainless steel works, satisfactory nitrided layer formation is almost impossible even if passive surface coat layers are completely removed prior to charging into a treatment furnace by cleaning with a hydrofluoric acid-nitric acid mixture. Uneven nitriding occurs not only in gas soft nitriding but also in nitriding of nitriding steel or stainless steel with ammonia alone (gas nitriding). Furthermore, in the case of works complicated in geometry, for example gears, even when they are made of ordinary structural steel, it is a fundamental problem that there is a general tendency to uneven nitriding.
The means or methods so far proposed for solving the above-mentioned essential problems encountered in gas nitriding and gas soft nitriding include, among others, the one comprising charging a vinyl chloride resin into a furnace together with works, the one comprising sprinkling works with chlorine, CH.sub.3 Cl or the like and heating at 200-300.degree. C. to thereby cause evolution of HCl and prevent oxide formation and remove oxides therewith, and the one comprising plating works in advance to thereby prevent oxide formation. Practically none of them have been put into practical use, however. Where chlorine or a chloride is used, chlorides such as FeCl.sub.2, FeCl.sub.3 and CrCl.sub.3 are formed on the steel surface. These chlorides are very fragile at temperatures below the nitriding temperature and can readily sublime or vaporize, damaging furnace materials badly. In particular, CrCl.sub.3 can sublime very readily, so that Cr deficiency may readily result in addition to the drawbacks mentioned above. Furthermore, the handling of the above-mentioned chlorides and the like is troublesome, although they are effective to some extent in preventing oxidized layer formation. Thus, none of the methods mentioned above can be said to be practicable. Accordingly it is an object of the invention to provide a method of nitriding steel by which a uniformly nitrided layer can be formed on the steel surface without unevenness in nitriding.